Steel with a composition of iron, carbon, silicon, phosphorus and molybdenum

ABSTRACT

A steel composition containing iron, carbon, silicon, phosphorus and molybdenum, comprised of about 0.85-0.95 percent by weight carbon, up to about 0.1 percent by weight silicon, up to about 0.015 percent by weight phosphorus, about 0.20-0.4 percent by weight molybdenum, with the remainder percent by weight being iron.

FIELD OF THE INVENTION

The invention pertains to a new steel composition with an improvedstrength property, composed of iron, carbon, silicon, phosphorus andmolybdenum.

BACKGROUND OF THE INVENTION

A similar composition of steel is known under the designation SKF3; theSKF3 steel is composed of 0.9-1.10 percent by weight carbon, 0.25-0.4percent by weight silicon, maximum 0.025 percent by weight phosphorusand maximum 0.10 percent by weight molybdenum, in addition to iron. TheSKF3 steel is commonly used to manufacture roller bearings.

The strength of steel is a major mechanical property. It is especiallyimportant in roller bearings, as it has great significance in thedurability of bearings. Although it is possible to use the known SKF3steel to make bearings with good mechanical properties, by increasingthe strength of the steel, the durability of the bearings would likewisebe increased. This is especially important for applications in aviationand space exploration and in heavy-duty gear boxes, lathes and the like.In applications such as these, the life span of the roller bearings isof crucial significance with regard to safety and security. The lifespan of the roller bearings is directly impacted by the durability ofthe bearing. The durability of the roller bearing is dependent upon thestrength properties of the steel. Therefore, the increased strengthproperties will increase the life span of the bearings and result ingreater safety and security.

Consequently, there is an urgent need for a type of steel with improvedstrength performance for the manufacture of roller bearings to be usedsuccessfully in technical areas requiring great safety, reliability andlimited maintenance of the bearings in heavy-duty systems.

SUMMARY OF THE INVENTION

The objective of the invention is to produce a steel composition withincreased strength capabilities to be used in roller bearings with theresult of greater durability of those bearings.

For this purpose, the invention concerns steel which is comprised ofiron, about 0.85-0.95 percent by weight carbon, about 0.1 percent byweight silicon, about 0.015 percent by weight phosphorus, and about0.2-0.4 percent by weight molybdenum. The strength properties of thesteel described herein proved to be much better than those of knownsteel for bearings.

Due to these remarkably improved strength properties, the steelpresently described is very suitable for the manufacture of high-graderoller bearings. The steel is particularly useful in technical areaswhere reliability, safety and durability of bearings is important, asmentioned previously. In this context, the application of a greaterstrength steel is most notable in high grade roller bearings used inaviation and space exploration, in heavy-duty gear boxes of heavyvehicles, and in heavy-duty high-speed lathes.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects of the present invention and the variousfeatures and details of the operation thereof are hereinafter more fullyset forth with reference to the accompanying drawings, where:

FIG. 1 is a comparison graph showing the relative strength of the MMMsteel as compared to the SKF3 steel. Results of a so-called slow bendingtest were plotted along the graph. The slow bending strength is plottedas a function of the tempering temperature after martensite hardening at860° C. (for 20 minutes) and oil quenching at 50° C.

FIG. 2 is a graph showing the results of durability tests performed onroller bearings manufactured from the MMM steel according to theinvention; and roller bearings manufactured from the known SKF3 steel.The graph shows the probability of deviation as a function of the numberof revolutions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferably, the steel according to the invention will contain about 0.88percent by weight carbon, about 0.05 percent by weight silicon, about0.0075 percent by weight phosphorus and 0.30 percent by weightmolybdenum, with the remainder percent by weight being iron.

The strength of the new steel according to the invention, referred to inshort as MMM steel or steel, was compared to the strength of the knownSKF3 steel. For this purpose, both types of steel were subjected to aso-called slow bending test, whereby grooves were cut in steel platesprior to the test.

In principle, the test was performed according to the proceduresdescribed in ASTM E 812-81; the ground test pieces were 10×10×55 mmlong. Grooves were cut in the test pieces by means of a fine grindingdisc. The grooves were 0.15 mm wide and 0.15 mm deep.

A triple point bend test clamp was used; the friction effects were keptto a minimum through the use of bearing rollers with a 40 mm expansionlength. Pressure was applied with a tensile strength machine; themaximum load was used for the computation of the breaking strength ofthe material in MP a units, etc. This value represents the apparentstrength of the material in the presence of a groove with a specificshape, i.e., the strength relative to the energy required for unstablecrack propagation.

The results are shown in FIG. 1, where the slow bending strength isplotted as a function of the tempering temperature, after martensitehardening at 860° C. (for 20 minutes) and oil quenching at 50° C.

The figure shows clearly that the strength of the new steel according tothe invention, i.e., the MMM steel, is significantly improved comparedto the strength of the known SKF3 steel.

Finally, roller bearings manufactured from the MMM steel according tothe invention and from the known SKF3 steel were subjected to thedurability test.

The bearings used for this test were commercially available 6205 DGBB(25 mm bore). To obtain the right kind of damage on the races, allbearings underwent a test run on a test installation for half an hour,with an impure oil lubricant, a load of 1.4 kN and an axial speed of5000 rpm. The impurity consisted mainly of metal with some silicondioxide filings derived from used gear boxed. The particles did have aspecific size distribution of up to maximum 40 μm. The concentration ofthe particles in the lubrication system was 40 mg/l. After the test runthe bearings were taken apart, cleaned, and reassembled. Subsequently,the durability tests were performed by means of so-called R2 durabilitytest machines, with a purely radial load of 5 kN and an axial speed of6000 rpm. The test bearings were operated at a temperature of 53° C. andwere lubricated with a "pure" mineral oil of the commercially availableShell Turbo T68 type.

The bearings were tested in such manner that a preset vibration levelwas exceeded, i.e., the bearing life was exceeded. The present level waschosen so that any development of surface unevenness due to fatigue ofthe race could be detected. After subsequent examination of the innerrace it appeared that the main type of damage was a splintering of theraces; the splintering started at impact sites generated during the testrun with impurities.

Subsequently, the life span of the bearings was assessed with the aid ofWeibull statistics.

The results are presented in FIG. 2. FIG. 2 shows the probability ofdeviation as a function of the number of revolutions. The figure showsclearly that the MMM steel according to the invention has asignificantly lower risk of failure than the known SKF3 steel for thesame number of revolutions, which means that the durability of MMM steelis significantly better than that of the known SKF3 steel.

While particular embodiments of the present invention have beenillustrated and described herein, it is not intended to limit theinvention and changes and modifications may be made therein within thescope of the following claims.

I claim:
 1. Steel with a composition consisting of about 0.85-0.95percent by weight carbon, up to about 0.1 percent by weight silicon, upto about 0.015 percent by weight phosphorus, about 0.20-0.4 percent byweight molybdenum and the remainder by weight being iron.
 2. Steel witha composition according to claim 1, comprising 0.88 percent by weightcarbon, 0.05 percent by weight silicon, 0.0075 percent by weightphosphorus, 0.25 percent by weight molybdenum and the remainder percentby weight being iron. .Iadd.
 3. Steel with a composition consistingessentially of about 0.85-0.95 percent by weight carbon, up to about 0.1percent by weight silicon, up to about 0.015 percent by weightphosphorus, about 0.20-0.4 percent by weight molybdenum and theremainder by weight being iron. .Iaddend.